In an automobile equipped with a hood at its body front, the hood of the automobile comprises: an outer panel; an inner panel; and other reinforcing members. Generally, in order to secure rigidity and strength of the hood itself, the panel structure uses a closed sectional structure in which the outer panel and the inner panel are joined together such that spaces are formed between them.
With respect to this type of hood, proposed are structures in which, near a central part of the inner panel, there are provided a plurality of beads having U-shaped cross section extending substantially parallel to each other so as to secure dent resistance and panel stiffness required for the hood and to raise a pedestrian-protective performance in the event of a frontal crush (Patent Documents 1 and 2).
The pedestrian-protective performance in the event of a frontal crush is currently evaluated by an HIC value computed from the acceleration-time waveform at the impact. The smaller the HIC value is, the less likely the injury to the pedestrian head occurs at the impact. For this reason, as described earlier, on condition that the dent resistance and panel stiffness are secured, there is a need for a hood structure capable of reducing the HIC value.
As for acceleration waveforms at the impact of the head on a part near the center of a hood, in general, a first acceleration wave is generated at the impact of the head on the hood at an early stage after the collision. Then, a second acceleration wave is generated when the hood part pressed by the head is moved in the lower direction of a vehicle to impact on built-in components, such as an engine arranged below the hood.
In order to reduce the HIC value, it is effective in particular to make the second acceleration wave small. For that purpose, it is necessary during a head impact to absorb the collision energy before the hood contacts built-in components and it is desirable to secure a sufficient clearance between the hood panel and the built-in components.
However, when considering that required parts for the automobile are installed in a limited space inside the hood, it is often difficult to secure the sufficient clearance between the hood panel and the built-in components.
Accordingly, such a hood structure is desired as the one in which the first acceleration wave at the impact of the head is allowed to be as large as possible to increase the energy absorption amount at an early stage after the impact to reduce the deformation stroke or the second acceleration wave generated by the collision of the hood with built-in components is allowed to be small.
According to the above hood inner structure in which a plurality of U-shaped beads are installed in parallel to each other, by providing the beads, flexural rigidity of the hood panel increases, flexural deformation at the impact of a pedestrian's head is suppressed, and impact loads can be dispersed. Therefore, the area and weight where the displacement is caused by the impact of the head are increased. Accordingly, as compared to the case where the beads are not provided, the first acceleration wave at the impact of the head is made larger. Moreover, since the beads are arranged substantially parallel to each other, deformation is liable to occur at the time of contact with built-in components, and there is an advantage that the second acceleration wave can be made small.
However, the recent demand for pedestrian protection is becoming more and more severe, and further improvement in pedestrian-protective performance is desired.
Therefore, in the above hood inner structure having beads of the U-shaped cross section also, with the aim of further improving the pedestrian-protective performance and in consideration of actual collision conditions, there are provided structures in which the height of the U-shaped cross section is varied according to a part and in which a flat seat is provided in a middle part of the vertical wall of the U-shaped cross section (Patent Documents 3 and 4).
As described earlier, under the condition that the clearance between the hood panel and built-in components is hard to secure sufficiently, in order to reduce the HIC value, it is necessary to further improve the first acceleration wave and reduce the second acceleration wave at the same time. That is, when the clearance between the hood panel and the built-in components becomes smaller, in order to meet the target requirements such as laws and regulations, it is necessary to further enlarge the first acceleration wave. Since the first acceleration wave is dependent on mass of an object to be collided with, it is desirable to enlarge a region where displacement takes place at an early stage after the impact of the head as much as possible. In order to do so, it is desirable for the stress to spread in a larger range at an early stage after the impact. In the case of the above bead structure of the U-shaped cross section, it is necessary to expand the stress propagation in a direction perpendicular to the length of the bead along which a force is not easily transmitted, and to expand the deformation range at an early stage after the impact.
Moreover, when the periphery of the collided portion is locally deformed, the deformation range at an early stage after the impact is reduced. Therefore, it can also be said that it should not be easily deformed locally by the impact load from the above of the vehicle. On the contrary, from the aspect of reducing the second acceleration wave, such a structure is desired that has a small deformation resistance against the load from below of the vehicle due to contact with built-in components and that is easily crushed. However, conventional technologies could not meet the demand of achieving both of them.    [Patent Document 1] JP-A No. 151159/2001    [Patent Document 2] JP-A No. 205866/2003    [Patent Document 3] JP-A No. 044542/2006    [Patent Document 4] JP-A No. 044543/2006